1. Field of the Invention:
This invention relates to improvements in a method for printing patterns or letters on substrates for printing (such as plastic containers) by thermal transfer of prints and to an apparatus useful in thermal transfer printing.
2. Discussion of the Background
Various methods have been used for printing on curved surfaces of various plastic containers used for cosmetic articles and foodstuffs, such as the offset method, the silk screen method, and the thermal transfer method.
Various disadvantages exist for each of these methods. The offset method has the disadvantage that since it operates by having different colors placed one after another on a rubber plate, the number of colors is limited and so-called process picture patterns using continuously changing colors cannot be printed. The silk screen method has the disadvantage that it suffers from a low printing speed in addition to its inability to print process picture patterns.
The thermal transfer method comprises printing a picture pattern in advance on a transfer sheet by gravure printing and transferring the printed picture pattern through the agency of heat. Therefore, it is capable of producing clean prints. It nevertheless has a disadvantage in that the printing speed is low because (1) the transfer relies for heat and pressure on the hot roll and (2) the transfer sheet is expensive because the base sheet is high quality paper. The transfer sheet conventionally used in the prior art for the thermal transfer method, as illustrated in FIG. 10, comprises a base sheet 21 made of quality paper, strips of a printing ink layer 3 formed as regularly spaced on the base sheet 21 through the medium of a peel layer 103, and an adhesive layer 105 applied on the printing ink layer 3. A frontal view of a transfer paper is shown (for example) in FIG. 3, which shows a transfer layout sheet 1 of continuous length made of a base sheet 21 on which printing ink surfaces 3a, 3b, 3c, etc. are coated at equal intervals, leaving a blank space 3 between each. By heating the transfer sheet with a hot roller kept at (for example) 220.degree. C. and, at the same time, pressing it against a given substrate for printing with the hot roller, the printing ink layer 4 is transferred in conjunction with the aforementioned adhesive layer 105 to the substrate.
Various apparatuses have been known for use in thermal transfer printing. A modified version of the machine shown in FIG. 1 has been known in which a sensor 13 is provided in the position indicated by the dotted line. In this figure, numeral 5 denotes the supplying roll axle around which the transfer paper 1 is wound, and numeral 6 denotes the takeup roll axle for taking up transfer paper 1a used in transferring, which is intermittently driven and rotated. An encouraging means (not shown) for rotating the supplying roll axle 5 reversely in relation to the direction in which the transfer paper 1 is pulled out is attached to the supplying roll axle 5. A clutch plate rotating the supplying roll axle 5 in the reverse direction by virtue of frictional force may, for example, be used as this encouraging means. This clutch plate encourages the supplying roll axle 5, being in contact therewith, to rotate constantly in the reverse direction in relation to the pull-out direction. This encouraging force is adjusted to be smaller than the pull-out force for the transfer paper 1 caused by rotational force of the takeup roll axle 6 and a heat roll 7. While the takeup roll axle 6 and the heat roll 7 are working, the clutch plate and the supplying roll axle 5 slip therebetween so that the supplying roll axle 5 rotates in the pull-out direction of the transfer paper 1.
The heat roll 7 for the heat transfer is usually installed to be movable upwards and downwards and is driven to constantly rotate in the counterclockwise direction, and, when moved downwards, presses the transfer paper 1 against an object of transfer 8 to perform transference by virtue of its heat. The heat roll 7 is heated to approximately 200 degrees by a heating means such as an infrared heater, which is not shown in the drawings. The objects of transfer 8 are successively fed from a feeding path 9 and successively or intermittently moved beneath the heat roll 7, one after another, by being loosely inserted on mandrels which are rotatably provided in rotating plates 10 which rotates in a clockwise direction. After the heat transfer is finished, the object of transfer 8 is transferred through a feeding-out path 12 to a next step. The sensor 13, using a photoelectric tube, senses the ends of the printing ink surfaces 3a, 3b, etc. and commands the rotation of the takeup roll axle to stop. The sensor 13 has been placed above the moving path of the transfer paper 1 immediately below the heat roll 7 where the center of the blank space 4 is to be stopped. The heat transfer is performed by such a machine described in the following manner. That is, the roll of the transfer paper 1 is set on the supplying roll axle 5 and hung therefrom over the takeup roll 6, as indicated by a solid line in FIG. 1. Next, the takeup roll axle 6 is rotated and stopped in a position where the leading tip of the initial printing ink surface 3a closely approaches the position immediately below the heat roll 7. This position is identified by using the sensor 13. The heat roll descends and feeds out the transfer paper 1 towards the takeup roll, performing heat transfer onto the object of transfer 8. The transfer paper 1a which has undergone the transfer process gradually hangs down as indicated by a dotted line in FIG. 1. In the latter half period of the heat transfer process, the takeup roll 6 begins to rotate and wind up a part of the suspended transfer paper 1a. After the heat transfer is finished, the heat roll 7 moves upwards and winds up an amount of the transfer paper 1a corresponding to one pitch. The rotation of the takeup roll axle 6 stops when the sensor 13 has sensed the end of the printing ink surface.
The blank spaces 4 formed between the printing ink surfaces 3a, 3b, etc. of the transfer paper are provided for the purpose of avoiding deformation of figures and patterns on the transfer paper due to any stretching thereof near the heat roll which might be caused by the heat. This blank space of the conventional art has a length of as much as 30 to 50 mm. Accordingly, there has been a problem in that the transfer layout sheet is greatly elongated and its cost is thus high.